1. Field of the Invention
The present invention relates to a method and apparatus for image forming. In particular, the present invention relates to a method and apparatus for electrophotographic image forming capable of effectively performing an evenly charging operation.
2. Discussion of the Background
Generally, an image forming apparatus includes a charging unit for charging an image bearing member such as a photoconductive element during an image forming process. While a non-contact type charging unit such as a scorotron charger, corotron charger or similar charger that does not contact the image bearing member has commonly been used, a contact-type charging unit is increasingly used because the non-contact type charging unit produces a large amount of undesirable discharge products including ozone. Among some different contact-type charging units available today, a charging unit having a charging roller pressed against the image bearing member is extensively used. For example, a charging roller whose surface is implemented by rubber or resin has been used. However, a charging unit using a charging member has a problem that toner and impurities accumulate on the surface of the charging member little by little and make charging irregular, thereby reducing a life of the charging unit.
To solve the above-described problem, a technique has been proposed in which a charging unit is provided with films wrapped around and adhered to opposite end portions of a charging member over the entire circumference and has a contact with an image bearing member to form a predetermined gap between a center portion of the charging member and the image bearing member. In this configuration, the center portion of the charging member does not contact the image forming area of the image bearing member and is therefore free from accumulation of smears, so that the life of the charging unit is prevented from being reduced. The films, however, start peeling at seams in the circumferential direction of the charging member due to repeated contact of the charging member and the image bearing member.
Another technique has been proposed in which the charging member employs a resin material instead of an elastic material such as a rubber and sponge. In other techniques, inorganic fine particles are dispersed on a surface of an organic image bearing member or siloxane cross-linking resin is used so that a protective layer is formed on a surface of the organic image bearing member to increase its abrasion resistance and mechanical strength.
However, a charging member that has a roller shape and made up of a rubber material has difficulty in cutting with high accuracy and causes high thermal expansion, thereby causing fluctuation of gap due to environmental changes. On the other hand, a charging member made up of a roller-shape resin material has a high degree of hardness so that its cutting operation can easily be performed with high accuracy. When a gap forming member is formed by a film member wrapped around both ends of the charging member, however, the hardness of the charging member may cause problems that the film is abraded with age, and that toner is agglomerated to an adhesive agent come out of end of the film. When an image bearing member is made up of an organic material, the image bearing member may be damaged at a predetermined point where the image bearing member is held in contact with the film member.
To solve the above-described problems, some techniques have been proposed that a charging member has rollers mounted on both ends of the charging member to form a gap between the charging member and an image bearing member. That is, a pair of gap forming members are held in contact with a non-image forming area of the image bearing member so that a photoconductive layer may not be deteriorated.
Referring to FIG. 1, a structure of a background charging unit disposed in contact with an image bearing member 215 is described.
In FIG. 1, the image bearing member 215 includes a tube 205 and a photoconductive layer 204 coated around an image forming area on a surface of the tube 205. That is, a non-image forming area of the tube 205 is left uncoated.
The background charging unit includes a charging member 214 and a pair of gap forming members 203. The charging member 214 includes a metallic core 201 and a resin layer 202 formed around the metallic core 201. The pair of gap forming members 203 are respectively arranged at both ends of the charging member 214. The pair of gap forming members 203 are held in contact with respective ends of the tube 205 of the image bearing member 215, at non-coated area of the both ends of the image bearing member 205.
With the configuration described above, however, leakage of a charge bias may easily be made to occur in the non-image forming area of the image bearing member 215 from the ends of the charging member 214, thereby a sufficient distance of gap needs to be maintained between the charging member 214 and the pair of gap forming members 203, as shown in FIG. 1. In this case, the tube 205 of the image bearing member 215 needs to be extended in a longitudinal direction, thereby causing the image forming apparatus to become large in size.